The disclosure of Japanese Patent Application Nos. 2000-054242 filed on Feb. 29, 2000 including the specification, drawings and abstract is incorporated herein by reference in their entirety.
1. Field of the Invention
This invention relates to a fuel cell having high reliability and high electricity generating efficiency. This invention also relates to a fuel cell having high cost performance due to an improved assembling process.
2. Description of the Background
Air pollution caused by exhaust gas from automobiles is a serious problem. Various types of electric vehicles have been introduced to reduce exhaust gas from automobiles. Yet electric vehicles have not been popular due to, for example, the necessity of frequent power charges and insufficient cruise range.
Recently, automobiles employing fuel cells have gotten a lot of attention due to these environmental concerns. Such fuel cells generate electricity by a reverse reaction of electrolysis using hydrogen and oxygen, they do not discharge anything except water. The most promising fuel cells for onboard use are of the solid polymer electrolyte type which can be operated at low ambient temperature.
The fuel cell includes a stack of many unit cells capable of generating a desired level of power. A unit cell comprises an electrode unit and separators. The electrode unit includes an electrolyte provided between two electrodes (i.e., a fuel electrode and an oxidant electrode). Each of the separators has gas flowing conduits to supply the fuel gas or the oxidizing gas. Each electrode unit is held between two of the separators. As the electrolyte for the solid polymer electrolyte type fuel cell, a solid polymer electrolyte membrane is employed.
Gas supply manifolds and gas discharge manifolds extend in parallel to a stacking direction of the stack. The gas supply manifolds supply the fuel gas and the oxidizing gas to the gas flowing conduits of the separators. The gas discharge manifolds discharge emission from the separators. Some separators have coolant flowing conduits to keep the fuel cell within a proper temperature range. A coolant supply manifold and a coolant discharge manifold are also provided in parallel to the stacking direction of the stack in order to circulate the coolant in the coolant flowing conduits of separators.
Usually, the gas supply manifolds for fuel gas and oxidizing gas, the coolant supply manifold and the coolant discharge manifold are formed by a series of connection holes provided in circumference parts of the separators. The output power of the fuel cell is taken from current collector plates that are provided at both ends of the stack. Insulation boards are provided at the outsides of the current collector plates. Pressing members are provided at the outsides of the insulation plates so as to hold the stack by pressure to the center of the stack.
A conventional fuel cell structure is disclosed in a Japanese Patent Laid-open Publication No. H09-266007 published on Apr. 4, 1997. In this prior art, the fuel cell comprises current collector plates and output terminals. The current collector plates are disposed in parallel with the unit cells and have hole portions to form supply and discharge manifolds.
A conventional fuel cell is also disclosed in a Japanese Patent Laid-open Publication No. H09-92324 published on Oct. 7, 1997. In this prior art, the fuel cell comprises supply-discharge members and current collection plates. The supply-discharge members are provided at the center of the stacking direction of the stack. The current collection plates have no hole portions to form supply and discharge manifolds. In this prior art, the fuel gas, the oxidizing gas and the coolant are supplied and exhausted through the supply and discharge manifolds which extend from a side of the stack and perpendicular to the stacking direction of the stack.
However, the conventional fuel cell disclosed in the publication No. H09-266007 has following disadvantages. Since supply and discharge manifolds extend through the hole portions of the current collection plates, respective parts of the current collection plates are exposed to the fuel gas having a reduction property, oxidizing gas having a oxidization property, or a coolant. Thus, the material of the current collection plate has to have excellent corrosion resisting ability.
Although carbon has excellent corrosion resisting ability and is typically used for a current collection plate, it is very expensive in cost and has higher electric resistance than metal. In case the current collection plate has high electric resistance, some of the electric power generated by the fuel cell is consumed at the current collection plate. Thus, the efficiency of electric power generation falls significantly. Besides, in case the material of the current collection plate is carbon, it is very hard to connect the electric wire to the output terminal of the current collection plate. Further, the output terminal of the carbon current collection plate may easily break since carbon is fragile material.
These disadvantages may be solved by employment of the metal for the current collection plate. However, the problem of corrosion of the metal collection plate would not be solved because the exhaust gas includes water that is formed by the reaction of hydrogen and oxygen on the electrode. Further, such corrosion of the current collection plate may be enhanced by electrochemical reaction due to the electromotive force generated in the fuel cell.
To solve the corrosion problem, the current collection plate may be coated with a corrosion resistive material such as gold. However, such gold coating is expensive. Besides, the long term durability of the collection plate is lessened since it is difficult to get rid of pin holes.
As an alternative way of solving the corrosion problem, it has been thought to cover the current collection plate by a gasket that seals around the connection holes. However, it is difficult to produce this type of the fuel cell due to the complicated structure of the gasket, so that the fuel cell becomes more expensive.
The conventional fuel cell disclosed in the publication No. H09-92324 has similar disadvantages. It does not have disadvantage relating to the output electric terminal since the current collection plates are not exposed to fuel gas, oxidization gas nor coolant. However, it is necessary to employ electric conductive materials for the gas supplying and discharging parts with the connection holes that are provided at the center of the stack, as viewed in the stacking direction.
Besides, the supplying and discharging parts in this type of fuel cell have to have connection holes extending in several directions to function. Accordingly, production of this fuel cell is extremely difficult due to its complicated structure. Thus, this type of fuel cell becomes too expensive.
Accordingly, an object of this invention is to solve above conventional disadvantages and drawbacks.
A further object of this invention is to provide a new and novel structure of a fuel cell with high reliability, high electric power generation and low production cost.
To solve the aforementioned disadvantages and drawbacks, the fuel cell of this invention includes a fuel cell formed as a stack of a plurality of unit cells, each unit cell including an electrode unit held by separators, each electrode unit including a solid polymer electrolyte membrane held by a pair of electrodes. Fluid passages extend through the stack in a stacking direction of the stack. An insulating supply-discharge member having a fluid passage communicates with at least one of the fluid passages extending through the stack. A current collection member is held between the supply-discharge member and the stack. An isolating element is positioned to fluidically isolate the current collection member from a fluid in the fluid passage of the supply-discharge member and from the fluid passages extending through the stack.
Since the current collection member is isolated from the corrosive fluid, the current collection member does not corrode, so that the reliability of the fuel cell can be increased.
In addition, since the current collection member need not have a sealing member, the structure of the fuel cell can be simplified and the cost of producing the fuel cell can be reduced. Besides, since the current collection member is small, the cost for producing the fuel cell can be reduced.
According to the fuel cell of this invention, an electric output terminal which is connected with the current collection member may protrude outwardly from the fuel cell while extending through the supply-discharge member.
Since the electric output terminal protrudes outwardly from the fuel cell by extending through the supply-discharge member, the electric output terminal can be protruded outwardly from the supply-discharge member even if the electrode is surrounded by the supply and discharge member.
A metal having excellent electric conductivity may be employed for the electric output terminal as well as for the current collection member. Thus, power loss of the fuel cell can be reduced and the electric power generation efficiency can be improved.
Since the electric output terminal may be made by a metal, its electric resistance can be reduced at the contact between the electric output terminal and external circuits. Further, a reliable fuel cell may be produced due to the durability of an electric output terminal made from metal.
According to a feature of the invention, the electric output terminal protrudes outwardly from the stack in the stacking direction of the stack.
Since the electric output terminal can be arranged near the center of the current collection member, a distance between the electric output terminal and the current collection member may be reduced. Accordingly, the cost for producing the fuel cell can be reduced due to the smaller current collection member.
Further, the structure of the supply-discharge member which extends through the electric output terminal can be simplified so that the production cost of the fuel cell can be reduced.
According to the fuel cell of this invention, the fuel cell includes a supply port having a flange port, an exhaust port having another flange port. These flange ports are abutted against the supply-discharge member so as to flow the fluid through the supply port and the exhaust port. The fuel cell further comprises a pressure member for pressing the supply-discharge member toward the center of the stack and for pressing the flange ports toward the supply-discharge member.
According to this invention, the supply port, the exhaust port and the supply-discharge member can be manufactured separately. In addition, the supply port and the exhaust port can be connected to the supply passage and the discharge passage respectively. Therefore, the best materials can be selected for the supply port and the exhaust port. Further, the cost for producing the fuel cell can be reduced. Further, since the flange port can be shaped easily with a corrosive resistant material such as stainless steel, the supply port and the discharge port can be produced at low cost.
Another aspect of this invention includes the supply-discharge member which comprises a sheath portion extending through the pressure member and housing the electric output terminal.
Since the electric output terminal is in the sheath having an insulating property, a metal having high strength and high rigidity can be used as the pressure member. The electric output terminal can therefore sustain the internal pressure of the fuel cell due to the fluid supply to the fuel cell.
Still another aspect of this invention includes the pressure member which comprises a protection portion covering at least a part of the supply-discharge member.
Since the supply-discharge member is covered by the protection portion, damage to the supply-discharge member due to increased internal pressure of the fuel cell can be prevented. Accordingly, the reliability of the fuel cell can be increased.